Market research firm, NPD Group, said late yesterday that on the whole, retail sales of game software, hardware and accessories dropped 17% last month to $1.03 billion. Software sales alone fell 23% to $510.7 million, which was more than analysts had expected. In addition, game hardware declined 8% to $391.6 million.

A year ago the game industry had some major releases to prop up this time period in the year. However, this year there have been only a few.

Nintendo’s DSi launched in the United States on April, with reports now that more than 800,000 units were purchased in the month. Other Nintendo DS unit sales accounted for an additional 200,000 units. Nintendo’s Wii came in third with 340,000 units sold, which is about 70% of the U.S. gaming hardware market.

Xbox 360 sold 175,000 units, just besting the PS2’s undisclosed figures (following the price reduction from $129.99 to $99.99), which were not far behind Xbox 360. PS3 sales totaled 127,000 units for the month after the quarter where Sony reported its first annual loss in 14 years.

On the software side, Nintendo ruled the roost with four of the month’s top selling games, including Wii Fit, Pokemon Platinum Version and the year-old Wii Mario Kart (a reportedly much easier-to-play version than prior Nintendo versions, due to the subtle Wii Wheel control).